There are a number of earlier proposals in the patent literature for elevating devices for automobiles or other motor vehicles, namely U.S. Pat. Nos. 1,525,477 (Hose), 1,944,351 (Landry), 1,962,924 (Bristol), 2,057,335 (Hott), 4,212,449 (Tsujimura) and 4,500,071 (Bagwell et al.).
The Hose patent is the oldest of these and provides a four cylinder lift. The problem of ensuring uniform motion of the hydraulic cylinders is not adequately addressed. It is mentioned that if there is uneven raising of the cylinders, then of necessity this will balance when the cylinders reach their upper limit, since all cylinders must necessarily then be of the same height. Manually operable stops are provided, but no details are given of the mechanism of operating them.
The Landry patent whilst primarily concerned with a master and slave cylinder arrangement for use on a vehicle does suggest on page three that the invention is applicable to "elevatable vehicle racks for chassis lubrication". Check valves are provided, which are intended to ensure that the motion of the two cylinders is uniform.
Both the Bristol and Hott patents are concerned with lifts particularly for buses. As such, they provide a complex in-ground arrangement. The Bristol patent has a mechanical arrangement for sensing any variation of the rails from the horizontal. In the Hott patent, a complex valve arrangement is provided to ensure that the two cylinders operate uniformly.
The Tsujimura patent discloses an apparatus including hydraulic cylinders, but is primarily concerned with the arrangement of swing arms for lifting a vehicle by its chassis or body.
The Bagwell et al. patent presents some difficulty in interpretation. So far as it can be understood, it provides a lift having two hydraulic cylinders, with a hydraulic circuit including velocity valves for safety. To connect the cylinders together, an exposed hydraulic line extends across the top of the apparatus. This hydraulic line has to be higher then the top of the highest vehicle when in a fully raised position. As a consequence, the whole apparatus has to be of a considerable height. Additionally, the necessary lifting range is only achieved by effectively doubling the travel of each hydraulic cylinder with a chain arrangement.
The principle of a pair of hydraulic cylinders arranged in a master-slave relationship is well known, with the fluid displaced by the travel of the master cylinder being passed to the slave cylinder to actuate it. Thus, U.S. Pat. Nos. 2,616,265 (Wilson), 2,765,626 (Ashley et al.), 3,143,924 (Pearson et al), 3,184,920 (Lohbauer et al), 3,476,016 (Dixon et al) and 4,655,031 (Kucera) all disclose apparatus including a master-slave cylinder relationship. The Wilson patent is particularly concerned with ensuring that the master and slave cylinders move uniformly together To this end, a somewhat complex circuit is provided. The example shown is for an hydraulic press. Similarly, in the Ashley et al patent, an hydraulic mechanism is provided, including a somewhat complex arrangement of valves for controlling the motion of the master and slave cylinders. This again is in relation to an hydraulic press. Similarly, the Pearson et al patent is concerned with a control arrangement for master and slave cylinders of an hydraulic press.
The Lohbauer et al patent is concerned with a different field, namely the leakage control for a bulldozer pitch jack circuit. As such, it provides pilot-controlled valve for controlling the incoming pressure to the cylinders.
The Dixon et al patent discloses a scissor jack including master and slave cylinders. Spring-loaded check valves are provided for overcoming any imbalances that occur.
In the Kucera patent, there is disclosed a phasing circuit for serially connected hydraulic pistons, particularly of the type employed for the adjusting the fore and aft reel position of a combine harvester. At each end of each cylinder, there is provided in series a flow-restricting orifice and a check valve, parallel to the inlet connection.
A variety of hydraulic control mechanisms are found in U.S. Pat. Nos. 2,331,108 (Ganahl), 3,355,993 (Williamson), 3,603,210 (Florjancic) and 3,703,849 (Renner et al). These are not concerned with master and slave cylinder arrangements. An unusual hydraulic cross-regenerative circuit is disclosed in U.S. Pat. No. 2,940,262.
A variety of hoists are disclosed in U.S. Pat. Nos. 1,688,607 (Thielen), 2,644,307 (Blair) and 2,909,358 (Southerwick). These show that the use of hydraulic or pneumatic cylinders for hoists.
The U.S. Pat. No. 3,173,659 discloses a safety lift with an unusual catch arrangement on the cylinder itself.